Pitch production, the making of a high softening point material by inducing thermal polymerization of normally liquid streams, is an ancient process.
Use of pitch, for sealing baskets of reeds floating in the river, or for sealing Noah's ark, is reported in the Bible. “Make thee an ark . . . pitch it within and without with pitch.” Genesis 8 14.
While some commentators believe Noah used naturally occurring petroleum seeps, others believe that the pitch referred to was wood tar pitch, made by taking the sap of trees and heating in a metal kettle, to drive off volatile components and induce thermal polymerization in the remaining liquid fraction. By such processing, the ancients could produce a pitch material which would have had both significant preservative and waterproofing properties. For purposes of this patent specification, it will be presumed that Noah used pitch derived from wood tar.
With the rise of great sailing ships, made of wood, use of pitch increased. Pitch was made from sap, from charcoal and from the roots of pine trees. Pine tar was used so extensively on ships that sailors were often called “tars”, in reference to the constant contamination of their feet with tar due to use on decks and line. From 1720 to 1870, North Carolina was the world's leading producer of naval stores, turpentine, pitch and tar, all made from the state's abundant pine trees.
While wood tar pitch was the primary pitch product for millennia, it gradually was displaced in importance by pitch derived from coal and, eventually, from petroleum. Although these materials (trees, coal and petroleum) may seem very different, they all provided a suitable starting point in a process to make pitch. Some of the similarity, despite very different starting materials, can be inferred from the definitions of wood tar and coal tar, reviewed next.
Wood tar is defined, by the Encyclopedia Britannica, online version, as “liquid obtained as one of the products of the carbonization, or destructive distillation, of wood.” Although wood tar based pitch was probably the first pitch product, it is also possible to produce pitch from coal. Coal tar is defined by Britannica as a “principal liquid product resulting from the carbonization of coal, i.e., the heating of coal in the absence of air, at temperatures ranging from about 900 to 1,200° C.” Coal tar pitch is made from coal tar. In addition to wood and coal based pitch products, liquid petroleum fractions can be converted into tar or petroleum pitch.
All pitch processes are similar. All start with, or produce as an intermediate product, a relatively low molecular weight liquid material. Cooking pine produces pine tar, with further heating producing wood tar pitch. Cooking coal produces coal tar, with further heating, or at least fractionation, producing coal tar pitch. When a heavy, aromatic refinery bottoms stream is heated sufficiently to induce thermal polymerization, petroleum tar and, eventually, petroleum pitch is formed. In some pitch processes, e.g., production of coal tar pitch, the intermediate light liquid phase may not observed or recovered as a separate product. Thus coal can go from coal to coke plus coal tar pitch plus intermediate products. The inherently high temperatures used in coal coking destructively distill the coal, simultaneously freeing normally liquid coal components and inducing their thermal polymerization into coal tar pitch fractions.
Little wood tar pitch is made or used today, except in specialized circumstances like reconstruction of tall ships or where other sources of pitch precursors are not readily available. Coal tar pitch is widely used for roofing, coatings, in anodes and for myriad other applications, but there are concerns about carcinogens, both released to some extent during the manufacturing process and in the finished product. Some states bar sales of some coal tar based products, because of concerns about toxicity. Petroleum pitch is commercially available and can be used for many of the coal tar pitch applications.
Processes making pitch from wood and coal sources typically use controlled combustion to heat a solid containing a pitch precursor. Controlled combustion, or oxygen injection during pitch manufacture, can degrade the quality of the product, in addition to burning some of it. Use of a hot metal surface to complete thermal polymerization, e.g., metal pot on an open fire as in Noah's time or a coil of metal inside a fired heater, protects the liquid pitch precursor from the fire but suffer from other drawbacks. It is hard to control and limit thermal polymerization reactions—once started; polymerization increases the viscosity and melting point of the polymerized material. The thickest parts of the partially polymerized pitch will be near the hottest surface. Hot spots in the pot or in the tubes of the fired heater can over polymerize the liquid feed, leading to a super thick, sticky material which rapidly cokes to form solids. As the pitch gets thicker, it is harder to move it off of a hot metal surface, so coking or fouling of the hot metal surface is likely to occur.
There are additional problems associated with making a binder pitch, with a softening point of 225-250° F., especially when attempts are made to produce this material from a petroleum source rather than coal tar. To make pitch from a petroleum fraction or any other light material, it is necessary to start with something having a relatively low molecular weight and heat it sufficiently to induce thermal polymerization. The high temperatures which induce thermal polymerization also lead to coking, with the coke clogging the plant and contaminating the product. In the early 1960's, a patentee reported that “Because of the stringent requirements, commercial pitch binders have been almost exclusively made from selected coal tar products.” U.S. Pat. No. 3,140,248, Jul. 7, 1964. That patentee, a petroleum refiner, reported several old “tricks” used to make high softening point material, which were reported not to work when binder pitch was desired, and a new trick which was alleged to work.
The “old” methods of making binder pitch started with catalytic cracking, to produce an aromatic rich bottoms material, limited thermal cracking of this aromatic rich material to produce “thermal asphalt”, followed by “soaking” for 3 to 5 hours in a soaking tank. While this approach produced pitch, the pitch was contaminated with coke and the soaking tank coked up. The improvement of the '248 patent was a continuous process. The aromatic rich feed was still thermally cracked to produce a “thermal asphalt”, but the thermal asphalt was then upgraded in a continuous process utilizing “short residence times and high lineal velocities” to make binder pitch. Thermal asphalt was upgraded to pitch in a soaking coil, in a furnace operating at carefully controlled conditions, including a residence time of at least about 4 minutes and no greater than 20 minutes. By using a flowing coil for “soaking” and limiting the soaking time to minutes instead of hours, it was reported possible to make pitch product with satisfactory properties.
The problems associated with making high softening point pitch products, products with a softening point above 250° F., are even more severe. Pitch producers try to compensate for the inherent instability of intermediate pitch products by operating at a vacuum (to reduce the temperatures required to remove volatiles) and/or operating with a wiped film evaporator, which relies on thin films and brute force mechanical wiping to prevent the pitch from staying for a long time in contact with a hot metal wall.
At this point, a detailed review of several pitch processes will be made, to show the state of the pitch manufacturing art in recent decades.
U.S. Pat. No. 2,752,290, assigned to Cabot, disclosed a continuous process for making pitch.
U.S. Pat. No. 2,768,119, filed Dec. 31, 1952, assigned to Phillips Petroleum, taught making petroleum pitch. An aromatic extract was prepared by solvent extraction, then thermally cracked to produce a fuel oil fraction from which pitch was recovered by vacuum distillation. The patentee reported that pitch could be made from petroleum and had many of the properties of coal tar pitch. The vacuum distillation conditions included a “pressure of about 1 mm Hg, a temperature in the range 440° F. to 650° F.” Presumably the vacuum distillation step was used to remove sufficient volatile matter to produce a product with the desired softening point (188° F. to 240° F. reported in the patent) without rapidly coking the distillation apparatus.
U.S. Pat. No. 2,992,181, assigned to Sinclair Refining, disclosed making petroleum pitch.
U.S. Pat. No. 3,140,248, filed Mar. 6, 1962, assigned to Socony Mobil, discussed above, taught making binder pitch by thermal cracking at 850 to 1050° F., at pressures of 250-900 psig, to produce “thermal asphalt” having a softening point of 130 to 170° F. The thermal asphalt passed through a continuous soaking zone maintained at 940 to 1020° F., with a liquid residence time of 4 to 20 minutes, preferably 7 to 15 minutes. The soaking zone operated at 30-400 psig, preferably 100-200 psig, to limit formation of excess coke in the pitch binder product.
U.S. Pat. No. 3,692,663, assigned to Osaka Gas, taught heating a tar fraction to 320-470° C. to make gas oil and pitch.
U.S. Pat. No. 3,928,170 taught injecting hot gas into heavy oil to make pitch.
U.S. Pat. No. 3,974 and U.S. Pat. No. 4,026,788, McHenry, taught pitch manufacture with inert gas sparging.
U.S. Pat. No. 3,976,729 and U.S. Pat. No. 4,017,327, Lewis, taught making pitch with agitation during heat treatment.
U.S. Pat. No. 4,039,423, assigned to Gulf Oil, taught heating, flashing and “oxy-activation” to make pitch.
U.S. Pat. No. 4,066,737, assigned to Koppers, describes an oxidative pitch process, which was part of a method of making carbon fibers.
U.S. Pat. No. 4,242,196 assigned, inter alia to Sumitomo Metal, taught heating a resid to 450-520° C. in a tubular heater for 0.5-15 minutes, then passing an inert gas at 400-2000° C. for direct contact heating for ½-10 hours, to make pitch.
U.S. Pat. No. 4,243,513, assigned to Witco, taught treating clarified slurry oil at 390-410° C. for 2+ hours, under reflux, to make pitch.
U.S. Pat. No. 4,340,464, assigned to Sinclair Refining, Method for Thermal Cracking of Heavy Petroleum, taught how to make pitch.
U.S. Pat. No. 4,431,512, assigned to Exxon, taught heat soaking steam cracker tar middle distillate at 420-440° C. for 2-6 hours, then vacuum stripping. Their U.S. Pat. No. 4,427,530 disclosed a similar process, using FCC bottoms as feed.
U.S. Pat. No. 4,522,701, assigned to DuPont, taught making pitch by heat soaking FCC residue fractions.
U.S. Pat. No. 4,673,486 taught treating a solvent deasphalted fraction with a carrier gas and thermal cracking at 400-600° C. to produce gas oil and pitch products.
U.S. Pat. No. 4,961,837, assigned to Intevep, Caracas, VE, taught making petroleum pitch for use as pitch binder.
U.S. Pat. No. 4,999,099 taught use of an oxidative purge gas to make pitch. An FCC heavy resid fraction was heat soaked at 3850° C., then subjected to an O2+N2 sparge.
U.S. Pat. No. 5,540,832, assigned to Conoco Inc., taught making mesophase pitch from refinery decant oil residue by heat soaking at 386° C. for 28 hours with N2 agitation.
Ashland Petroleum has a series of patents on high softening point pitches, primarily for manufacture of carbon fiber. Their U.S. Pat. No. 4,671,864 taught vacuum flashing, or use of a wiped film evaporator, to reduce residence time of pitch at high temperature and make pitch having a softening point of about 250° C. U.S. Pat. No. 5,238,672 taught heating isotropic pitch with inert gas, at high temperature, to make mesophase pitch. U.S. Pat. No. 5,316,654 taught use of a wiped film evaporator (WFE) to make high softening point pitch. U.S. Pat. No. 5,429,739 taught use of reduced pressure and partial oxidation, converting a conventional 250° F. softening point pitch to a higher softening pitch in a WFE. The conventional output from a WFE was low, partial oxidation sped up the process. U.S. Pat. No. 5,614,164 taught use of a WFE to make mesophase pitch. The process started with a pitch with a softening point of 93-233° C., processed this in a WFE for 115-300 seconds to produce “enriched pitch” with a 5% maximum mesophase content, then stripped with an inert gas for up to 18 hours to produce the desired pitch product, with a softening point of 177-399° C.
The Eureka® Process, jointly developed by Kureha Chemical Industry Co. Ltd and Chiyoda, has been used for over 20 years to make pitch products. The process reduces the cracked oil partial pressure by injecting steam into the pitch forming reactor. Steam injection also helps keep the molten pitch as a homogeneous liquid.
Although not related to pitch production, mention will be made at this point of use of molten metal baths, for metal plating, to make float glass and to dry paper pulp, in U.S. Pat. No. 5,619,806, Drying of Fiber Webs, Warren. The patentee used an alloy composition of bismuth and zinc.
All of the patents discussed above, and hereafter, are expressly incorporated by reference, in their entirety.
This review of industrial pitch processes shows work making pitch has continued for most of the last century. In addition to this historical patent work, primarily by major refiners or by pitch manufacturers, some work has been done recently at universities on new ways of making pitch, with most focus being on higher softening point pitches.
I reviewed these multiple routes to pitch products, especially to high softening point pitch products, and felt there was a need for a better way to make pitch. I did not want to have to burn some product to make it (oxygen or air injection as a heat source). I did not like the use of hot metal surfaces to heat viscous pitch products or precursors, these hot surfaces were cursed with a “Midas touch”, which produced coke, rather than gold. I especially wanted to avoid the high capital and operating cost of, and limited throughputs associated with, use of high vacuum and wiped film evaporator technology.
In reviewing the problems associated with this process, which has been around for millennia, I discovered a better way to heat the pitch precursors and/or the intermediate softening point pitch products, which completely avoided the problems associated with use of hot, solid metal surfaces to heat pitch and pitch precursors.
I realized that by using a technique and technology used for decades to make plate glass (forming glass on a bed of molten metal), I could overcome the heating barrier imposed by solid metal heating surfaces. I used a molten metal bath to heat the pitch precursors, pitch intermediates and/or final product.
The molten metal bath was wonderfully efficient at heating the feed to a sufficiently high temperature to induce thermal polymerization. Molten metal was relatively free of hot or cold spots, because of its high thermal conductivity. More important, neither the pitch precursors nor the pitch product would stick to the molten metal.
Molten metal also permits a flexible design approach, permitting injection of the metal into the oil or vice versa, though not necessarily with equivalent results. When pitch, or a pitch precursor, is injected into a molten metal bath, it is easy to increase or decrease process severity by changing the depth of molten metal in the bath, the temperature of the metal, the pressure in the molten metal bath or the presence of a stripping gas to create a “pseudo vacuum”, or some combination of these. For the first time, pitch producers have many more degrees of freedom to pursue the best pitch product, in a process which is wonderfully tolerant of mistakes. While mistakes may be made, the coke so generated will not stick to the molten metal, so the pitch plant can generally producing pitch even if some solids will be present. It is better to have a plant that continues to work, when making some off spec product, than a plant which shuts down with coke deposits.